make it so /etc/resolv.conf always has a search

It is not enough to substitute the search domain. If none exist, one
must be added so that even older versions of facter return something
when called with fqdn.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #105 from dachary/wip-domain

make it so /etc/resolv.conf always has a search